The properties of rigid polymer foams are important for the design and performance of sandwich structures that are used in a wide range of engineering applications, including airframes, transportation vehicles, boat hulls, radar systems, surf boards, and space structures (DIAB Inc. Technical Information for Divinycell® Foam, 2002; Rohm A G. Technical Information for Rohacell® Foam, 2002; Baltek Corp. Technical Information, 2002; Marsh J. Fire-Safe Composites for Mass Transit Vehicles. Reinforced Plastics, 46(9):26-30, 2002). Currently, most high-performance structural foams are fabricated by expanding (blowing) liquid polymers to form rigid, low-density foams. Some of the leading thermoplastic foams made in this way are polymethacrylimide (PMI) and partly cross-linked polyvinyl chloride (PVC), with trade names Rohacell® and Divinycell®.
Syntactic foams are composites consisting of hollow microspheres (minute hollow bubbles, microbubbles, or microballoons) that are dispersed in a resinous matrix, or binder. These microspheres are commonly made from inorganic materials such as glass and silica; and polymeric materials such as epoxy resin, unsaturated polyester resin, silicone resin, phenolics, polyvinyl alcohol, polyvinyl chloride, polypropylene, and polystyrene. One example of syntactic foam known in the art to be used as structural foam is sold by Novamax industries under the tradename “Novacore”. This product uses an epoxy as the continuous resinous matrix or binder phase.
In these syntactic foams, the resinous matrix is typically substantially non-cellular. However, because the microspheres are hollow, their inclusion reduces the density of the syntactic foam. Thus, essentially all of the reduction in density (relative to that of the unfoamed epoxy matrix) is attributable to the gas contained in the microspheres. In some instances the epoxy matrix is expanded slightly by incorporating into it expandable plastic spheres and/or thermally decomposable blowing agents such as azodicarbonamide or p,p-oxybis(benzene sulphonyl hydrazide).
Although one- and two-part epoxy-based syntactic foams have enjoyed some success as reinforcing foam in the auto industry, they suffer from some deficiencies. For example, epoxy-based syntactic foams are generally very brittle and thus lack fracture toughness. Therefore, these foams tend to shatter on impact (such as in a vehicle collision) or crack easily under stress.